set the category-price to posts with default-price

* set category-price to post with default-price when first time a price is assigned to a category
* moved pricing-types to constants in LaterPay_Helper_Pricing
* replaced own implementation of `get_category_by_name`  with WordPress API [`get_term_by()`](http://codex.wordpress.org/Function_Reference/get_term_by)
* fixed table-name in CategoryPrice-Model 

__References to Issue:__ #231
the name `$category_doesnt_exist` implies a boolean. It should be a positive `$category_exists` so `if` clauses don't turn into double negatives, but `empty` checks for a `string`? Or does `empty` also return `false` on `falsyness`?